They all come from an Indo-European root teut- meaning 'people, nation'. This survived into Old English as théod, and the derived form théoden 'lord' used by Tolkien, but was mainly replaced by the word folc 'folk' (and later of course by the French borrowing 'people').

But as well as being Norse þjóða (i.e. thjódha) and Gothicþiuda (thiuda), it was borrowed into Latin as Teutoni, the name of a Germanic tribe in Jutland, later extended more widely to the Germanic peoples.

In the Old High German form diutisc it gave rise to the German word deutsch, the Dutch duits, and the English Dutch. Until quite recently "Dutch" referred to both German ("High Dutch") and what we now call just Dutch (formerly "Low Dutch"). In the name Pennsylvania Dutch it still refers to German. The German and Dutch languages, and the intermediate Plattdeutsch or Low German form a dialect continuum: there is no hard and fast boundary between them.

The Irish tuath 'people' is a separate development from the same Indo-European root.